The present invention relates to a method of forming a web of fibres intended for use in absorbent products, such as sanitary towels, incontinence products, nappies or the like, by air-laying fibres. The invention also relates to a fibre web produced using this method.
Absorption bodies for use in disposable absorbent products of the type mentioned above are produced from what is known as fluff pulp made from cellulose. Fluff pulp comes in various forms, such as mechanical, thermomechanical, chemithermomechanical or chemical pulp. Fluff pulp is delivered in bales or rolls and is defibrated in mills before forming. The mat formation for forming absorption bodies is carried out by defibrated pulp being transported in an air flow towards a forming wire, usually in the form of what is known as a mat-forming wheel. This is air-permeable and the fibres remain on the periphery of the mat-forming wheel and form a fibre mat of low density, which fibre mat is compressed to the desired density in further processing.
During mat formation, a continuous web of fibre material can be formed, which web is compressed and cut to form individual absorption bodies. In further handling, these can be separated in the web direction for separate positioning between web materials running continuously in the web direction, which are intended to form a covering around separate absorption bodies to form individual products, such as nappies, sanitary towels, incontinence products or the like.
It is also common to combine the individual absorption bodies with a further absorption body of different density and extent. One purpose of having two or more absorption bodies of different density in an absorbent product can be to create a density gradient in the thickness direction of the product in order to control liquid transfer into the product. Another purpose can be that it is desirable to have a highly compressed spreading layer which has the capacity to spread received liquid over the length of the entire product for better utilization of available absorption material. This is because it is a common problem with absorbent products, such as nappies or the like, that the product becomes locally saturated with liquid and leakage occurs long before available absorption material is utilized. When two absorption bodies which are to work together are combined, however, it is necessary for the separate layers to cohere intimately so that an insulating interspace is not formed between the layers, that is to say areas in which the layers are separated and liquid transfer consequently does not take place effectively between the layers. If an insulating interspace is present between the two absorbent layers, the risk of leakage in the lateral direction of the product is of course great. In practice therefore, after two different layers of different density have been combined, it is necessary to compress the layers arranged one on the other. After the two layers of different density have been co-compressed in such a manner, the desired effects mentioned above are lost to a greater or lesser extent.
Mat formation can also be carried out by separate absorption bodies being airlaid in moulds arranged with a uniform mutual spacing over the periphery of the mat-forming wheel. Mat formation can then be effected either by continuous forming taking place on the mat-forming wheel and the fibre material outside the moulds being brushed away and fed back into the system or by the fibre material being guided only into the separate moulds. In the former case, the brushing can interfere with the absorption bodies formed and tear these. This problem is particularly marked if the absorption body formed is not homogeneous but there are irregularities caused by flock formation during forming. In such cases, large chunks or clumps can be torn out of the absorption bodies during brushing, so that the quality becomes variable, even to the extent that large hollows appear in the absorption bodies. When the fibre material is guided only to the moulds, there is a risk of great disruption in the air flow at the edge portions of the moulds, which can cause defects in the absorption bodies.
The development of mat-forming in moulds on mat-forming wheels has been going on for a very long time. As early as in 1970, U.S. Pat. No. 3,518,726 described mat-forming in moulds, where the air flow and the forming in the moulds is controlled by the bottom in the moulds having different hole density and/or different hole size in different areas, as a result of which the absorption bodies formed can have different thickness in different areas and/or different density in different areas. Already in this publication therefore, forming of anatomically adapted absorption bodies for use in, for example, sanitary towels is described.
It has been known for a long time, from inter alia the abovementioned publication, to have moulds with different depth in different areas for forming anatomically adapted absorption bodies. U.S. Pat. No. 4,598,441 deals with the problem that these absorption bodies with different depth in different areas formed in moulds on a mat-forming wheel have the limitation that one side of the absorption body formed is plane and thus not anatomically adapted. U.S. Pat. No. 4,598,441 proposes using two mat-forming wheels, where absorption elements with a profiled side are produced on each mat-forming wheel, after which the plane sides of absorption elements formed on one mat-forming wheel are applied to the plane sides of absorption elements formed on the other mat-forming wheel to create absorption bodies in which the entire outer contour is shaped. In this way therefore, optimally anatomically adapted absorption bodies can be produced.
Over the years, development has moved towards very complicated and large mat-forming wheels. This is due on the one hand to the fact that manufacturing speeds have increased and larger air flows are required for forming the absorption bodies and on the other hand to the fact that it became desirable to have absorption bodies with a number of layers. Mention was made above of the disruptive air flows which can occur in the edge areas of covers which separate the flow of airborne fibres from surrounding air. When a fibre layer is formed on top of a layer already formed in the moulds on a large mat-forming wheel, there is a great risk of disruptive air flows in the edge areas of the covers destroying the layer already formed. EP-B1-0292624, for example, describes a proposal for solving this problem. This document discloses a mat-forming wheel with two mat-forming covers arranged immediately after one another and with two negative-pressure chambers which interact with the covers. For the purpose of avoiding pressure differences at the transition between the two covers, the negative-pressure chamber for one cover extends in under the other cover. A disadvantage of this is that the fibre-forming in one mould is disrupted by the negative pressure from the negative-pressure chamber of an adjacent mould and that as a result the forming becomes uneven. Sealing problems are also described in U.S. Pat. No. 3,717,905.
U.S. Pat. No. 3,501,813 expresses the prejudice that using a number of mat-forming wheels for forming an absorption body with a thicker central part involves considerable disadvantages. This publication states that this procedure with a number of mat-forming wheels it too expensive on the one hand because separate forming units are required for forming each web and on the other hand because equipment is required for combining the webs to form the intended absorption body.
Forming absorption products with a varying basis weight and different thickness in different areas on a single mat-forming wheel is described in a great many patent publications. Examples of these are U.S. Pat. Nos. 4,016,628, 4,388,056, 4,761,258 and 4,859,388. In U.S. Pat. No. 4,761,258, the flow of fibres is controlled by arranging air holes in the forming wire in a selected pattern, in which way the weight per unit area of an absorbent product can be varied in different areas. U.S. Pat. No. 4,859,388 describes the forming of separate absorbent products on a large mat-forming wheel, said products also being compressed in the moulds on the mat-forming wheel. This compression is brought about by a separate, relatively large compression wheel which has a number of radially projecting tooth-like members which are arranged so as, when the two wheels rotate, to penetrate each mould on the mat-forming wheel to compress the absorption bodies formed.
Requirements for increasingly sophisticated absorption bodies in absorbent products, such as sanitary towels, nappies, incontinence products and the like, and efficient high-speed manufacturing have resulted in development moving towards large mat-forming wheels of very complicated construction which take up a great deal of space. The requirement for ever faster manufacturing speed has resulted in the forming length for forming the absorption web becoming increasingly long, which has in turn resulted in the need for larger forming wheels, which require a great deal of energy and, as mentioned above, space. With all the peripheral equipment associated with large mat-forming wheels, factory buildings are often too small, which results in increased investments in the form of building costs when changing over to new, higher-performance mat-forming wheels.
An example of a complicated mat-forming wheel is disclosed in U.S. Pat. No. 4,765,780. In this publication also, the prejudice emerges against using a number of separate mat-forming wheels for the reason that it is too expensive. Instead, a solution is proposed, in which fibres from a common mill are fed in individual fibre flows via separate lines to different forming covers along the periphery of a large mat-forming wheel, different layers being formed on one another as the forming wheel passes the different forming covers. It is stated in U.S. Pat. No. 4,765,780 that this forming installation is particularly suitable in the case of addition of what is known as highly absorbent material. As such highly absorbent material is more expensive than absorbent cellulose fluff pulp fibres, an advantage of the method and the arrangement according to U.S. Pat. No. 4,765,780 is stated to be that the highly absorbent material can be added to one of these fibre flows for positioning within a desired area in the absorption core which is built up on the mat-forming wheel. U.S. Pat. No. 4,765,780 also indicates using a second mat-forming wheel for producing what is known as a take-up zone in the absorbent product, that is to say a zone which is smaller in extent in relation to the length and width of the remaining part of the absorption body. This take-up zone is intended to be arranged directly in front of the genitals of the wearer during use of the product so as to be capable of rapidly taking up large quantities of liquid which are then conveyed onward to the actual absorption body, that is to say the part which is formed from different fibre flows on the first-mentioned mat-forming wheel.
Development is now moving towards increasingly thin absorption bodies, which means greater requirements for manufacturing accuracy in order for the absorbent products to function in the intended manner. Moreover, the demands of consumers for uniform quality and better functioning in terms of leakproofness, fit and comfort are increasingly exacting. Furthermore, requirements for better performance and increased price pressure owing to greater competition result in the manufacturing speed having to be increased considerably.
As mentioned above, increased manufacturing speed results in more exacting requirements in connection with the forming of the absorption web. In combination with demands for considerably thinner products, this results in known manufacturing methods not functioning satisfactorily in all respects.
The object of the present invention is to produce a manufacturing method which makes higher manufacturing speed possible at the same time as the manufacturing quality increases compared with previously known manufacturing methods.
To this end, the invention is characterized mainly in that separate air flows containing fibres are fed to a number n of different mat-forming wheels, where n is a whole number which is at least 2, in that separate web layers are formed on each mat-forming wheel, in that said intended fibre web is formed by virtue of said web layers being combined downstream of the mat-forming wheels to form a common fibre web which is imparted very great manufacturing accuracy by virtue of the manufacturing method, in that the manufacturing speed and thus the web speed is very high, and in that the desired manufacturing accuracy at the web speed concerned is achieved by selecting a sufficiently great number n of mat-forming wheels.
The manufacturing method according to the invention affords a number of important advantages. The investment costs decrease because small mat-forming wheels are much cheaper than large ones. Furthermore, operating costs are lower because less energy is required to drive a number of small wheels than one large one. The size of the wheels can be reduced by in the order of a factor of 5 if there are, for example, three small mat-forming wheels instead of one large one with the same capacity. The operating costs decrease further as a result of the fact that smaller quantities of air are required in the method according to the present invention compared with forming on a single wheel. When thin layers are formed on small wheels, the pressure drop can be low while, in the case of forming the entire absorption web on a single wheel, a high pressure drop is required in order to mat-form upper layer portions through layers already formed on the mat-forming wheel.
A further advantage of a number of mat-forming wheels arranged one after another is that other materials can be arranged between the web layers and that the method according to the invention is in this way considerably more flexible than conventional manufacturing methods.
The most important advantage of the manufacturing method according to the invention, however, is that the manufacturing accuracy can be increased in spite of increased manufacturing speed compared with mat formation on a single large mat-forming wheel. This is of particular importance when the trend is towards thinner products.
The fibre web is suitably formed by a number n of mat-forming wheels, where n is a whole number which is at least 3. When three mat-forming wheels are used, the manufacturing accuracy is increased by in the order of 50% compared with production on a single mat-forming wheel.
According to a particularly suitable embodiment, the fibre web is compressed in one or more step(s) after forming.
According to one embodiment, the invention is characterized in that at least one air flow has added to it a material in the form of fibres or particles which differs from the material in one or more of the other air flows, in which way a web containing different layers is obtained. The latter embodiment is suitably characterized in that said material in the form of particles or fibres consists of a highly absorbent material, and in that this is added to said air flow together with the first-mentioned fibres. This highly absorbent material in the form of particles or fibres is suitably fed in between at least one pair of adjacent mat-forming wheels by application of said fibres or particles to the web layer formed upstream.
One problem associated with the addition of highly absorbent particles to absorption bodies made of fibre layers is that the particles completely or partly fall through the fibre layer and therefore do not remain in the intended place. In the manufacturing method according to the invention, this can be entirely avoided by virtue of the fact that said web layer formed upstream is compressed before the application of highly absorbent material in the form of fibres or particles.
The method according to the invention is suitably characterized in that the fibre web is compressed to a fibre density of at least 200 kg/m3, preferably between 300 and 700 kg/m3. The method according to the invention is also suitably characterized in that the manufacture of the material web takes place at a speed in excess of 400 m/min. At said manufacturing speeds and degrees of compression, the method according to the invention is superior to previously known manufacturing methods. It is to be pointed out, however, that the production of fibre webs of lower density and/or at lower manufacturing speeds can also take place with greater accuracy using the method according to the present invention compared with conventional techniques.
It is characteristic of the manufacturing method according to the invention that the manufacturing speed of the web can be increased while retaining manufacturing accuracy in terms of variations in weight per unit area in the web formed by adding more mat-forming wheels. The method according to the invention is also characterized in that the manufacturing accuracy in terms of variations in weight per unit area in the web formed can be increased by adding more mat-forming wheels.
The term fibre density means that only fibres forming part of the fibre web are included in the calculation of the density, that is to say additions of, for example, highly absorbent particles to the fibre web do not increase the fibre density.
The method according to the invention is suitably characterized in that the combined fibre web is compressed to a stiffness in the dry state of in the order of 1-15 N measured according to ASTM D 4032-82. The method according to the invention is suitably characterized in that the dry-formed fibre web is, after compression, mechanically softened to the desired hardness. According to a suitable embodiment of the method according to the invention, the invention is characterized in that the dry-formed fibre mat is imparted the desired reduced stiffness and the desired extensibility by virtue of the degree of compression selected and the compression pattern selected.
A fibre web according to the invention for use as absorbent elements in absorbent products, such as sanitary towels, incontinence products, nappies or the like, is characterized mainly in that the fibre web is constructed from a number n of separate web layers, formed one after another on n different mat-forming wheels and combined to form a common fibre web, where n is a whole number which is at least 2, the desired manufacturing accuracy in terms of variation in weight per unit area of the common combined fibre web being achieved by selecting a sufficient number n of web layers formed on n separate mat-forming wheels.
Further suitable embodiments of the method and the fibre web according to the invention emerge from the subsequent patent claims.